To me if KD remains in OKC there are three super powers in the West, however were he to leave OKC and go to either GS or SA there would be only two, that would mean only one rival to contend with out West. So by leaving he actually improves his chances, there is also the possibility of Russ bolting somewhere else.
If the choice is between GS and SA, while joining the Dubs might give him the stronger possibility of winning the le, there is next to no chance that the Dubs would ever be seen as "his" team. Even if he could prove to be the best player it would be Curry's team much like the Heat always belonged to Wade. While as long as TD draws breath the Spurs would be seen as his, if KD were to join the Spurs, it is easy to envision him becoming the teams, top scorer. Which would enable Kiwi to concentrate on being the league's top lock down perimeter defender. The media could easily see the Spurs as KD's team, with strong contributions from KL and LA. While the "Servant" doesn't appear as ego driven as many NBA stars, being the 'man' might be of some importance to him.